1. Field of the Invention
This invention relates to light emitting devices with phosphor wavelength conversion and in particular, although not exclusively, to white light generating devices based on one or more light emitting diodes (LEDs). Moreover embodiments of the invention concern packaging arrangements for high emission intensity (i.e. ≧50 lumens light emission intensity or ≧1 W input power) white light emitting devices for general lighting applications.
2. Description of the Related Art
White light emitting LEDs (“white LEDs”) are a relatively recent innovation and offer the potential for a whole new generation of energy efficient lighting systems to come into existence. It is predicted that white LEDs could replace filament (incandescent), fluorescent and compact fluorescent light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor combined with the light emitted by the phosphor provides light which appears to the human eye as being nearly white in color.
An example of a light emitting device based on LEDs that can be operated from a high voltage (110/220V) alternating current (AC) supply is described in co-pending U.S. patent application Publication No. US 2009/0294780 published Dec. 3, 2009 and is shown schematically in FIG. 1. Referring to FIG. 1 the device 10 comprises a ceramic package 12, such as a low temperature co-fired ceramic (LTCC), having an array of nine circular recesses (cavities) 14 (FIG. 1 shows an array of nine recesses arranged in a square array 3 rows by 3 columns) in which each recess 14 is configured to house a respective LED chip 16, typically a blue emitting gallium nitride-based LED chip. The walls of the recesses 14 are inclined and can include a reflective surface 17 such as a metallization layer of silver or aluminum such that each recess 14 comprises a reflector cup for increasing emission of light from the device. The package 12 is a multi-layered structure and incorporates a pattern of electrically conducting tracks 18 configured to interconnect the LED chips 16 in a desired configuration (e.g. a serially connected string or a bridge configuration for a self-rectifying arrangement). The conducting tracks 18 are configured such that a part of them extends into the recess to provide a pair of electrode pads 20 on the floor of the recess 14 for electrical connection to a respective LED chip 16. On a lower face of the package 12 one or more solder pads 22 are provided for electrically connecting the device 10 to an AC power source. The solder pads 22 are connected to the conducting tracks 18 by conducting vias 24. Each LED chip 16 is mounted in thermal communication with the floor of the recess using a thermally conducting adhesive such as a silver loaded epoxy or by soldering. Electrodes 26, 28 on the LED chip 16 are connected by a bond wire 30, 32 to a respective electrode pad 20 on the floor of the recess. Each recess 14 is completely filled (potted) with a transparent polymer material 34 such as silicone which is loaded with the powdered phosphor material(s).
A problem with existing light emitting devices, in particular white light emitting devices intended for general lighting which require a high intensity output of ≈500-600 lumens or higher (i.e. an input power of about 6.5 to 8 W), is thermal degradation of the phosphor material with time which can result in a significant change in the correlated color temperature (CCT) and/or intensity of light emitted by the device. The inventors have further appreciated that absorption of water by the phosphor material(s) during operation of the device can also significantly affect the performance of the phosphor material(s) and hence the device. The effect of water absorption on photo luminescence varies between phosphor compositions and can be more pronounced for silicate-based phosphor materials which are able to more readily form water soluble compounds. Initial tests suggest that the absorption of water can occur even when the phosphor material is encapsulated in a polymer binder, such as a silicone, and a reduction in light emission of ≈10% may occur for a device with an ortho-silicate phosphor that is operated in a humid environment (i.e. ≧80% relative humidity) at a temperature of 25° C. for more than 200 hours. As well as phosphor degradation, other packaging materials can be affected by the presence of water such as for example the transparency of the encapsulating polymer materials, the reflectivity of reflective surfaces and the performance of the LED chip.